(1) Field of the Invention
The present invention relates to a phased array antenna system with two dimensional scanning. It is suitable for use in all areas of technology employing scanning phased array antennas, e.g. radar, television and radio broadcasting and telecommunications, mobile cellular radio (“mobile telephones”) in particular.
(2) Description of the Art
Phased array antennas are well known: the subject is discussed in detail in for example a standard textbook well known in the art of antennas, “Microwave Scanning Antennas”, R. C. Hansen, Vol 3 Array Systems, Academic Press, NY, 1966. Such an antenna comprises an array of individual antenna elements (usually eight or more) such as dipoles or patches. The antenna has a radiation pattern incorporating a main lobe or beam and side lobes. The centre of the main lobe is the antenna's direction of maximum sensitivity in receive mode and the direction of its main output radiation beam in transmit mode.
It is a well known property of a phased array antenna that delaying signals received by antenna elements by a delay which varies with element distance across the array, then the antenna main radiation beam is steered or tilted towards the direction of increasing delay. The angle between main radiation beam centres corresponding to zero and non-zero variation in delay, i.e. the angle of tilt, depends on the rate of change of delay with distance across the array. Delay may be implemented equivalently by changing signal phase (hence the expression phased array, albeit a specific value of delay corresponds to different phase shifts at different frequencies). The main beam direction of the antenna pattern can therefore be altered (referred to as “beam steering”) by adjusting the phase relationship between signals fed to antenna elements.
A conventional technique for beam steering by adjusting the phase relationship between signals fed to antenna elements is to provide a respective variable phase, shifter or variable delay for each antenna element. This provides control of each antenna element's signal independently of other antenna elements' signals. Equivalently, cascaded arrangements of variable phase shifters may be used in which each variable phase shifter provides a signal to a respective antenna element and to a respective variable phase shifter. Examples of the use of multiple variable phase shifters are disclosed by, for example, Japanese published Patent Application No. 04-320122 and U.S. Pat. Nos. 3,277,481, 4,242,352 and 5,281,974.
The use of variable phase shifters in numbers comparable with antenna elements is undesirable, because it greatly increases antenna design complexity and expense. A variable phase shifter is much more complex than a fixed phase shifter. This problem is particularly relevant to the case of a two dimensional phased array antenna which is required to scan in both dimensions: e.g. a phased array antenna consisting of a 64×64 array of antenna elements would require 4095 variable phase shifters and respective associated control circuitry.
The problem of excessive numbers of variable phase shifters has been addressed for the case of a one dimensional phased array antenna (e.g. a line of dipoles) scanned in a plane of the array dimension: the following published International Patent Applications disclose solutions to the one dimensional problem, WO 03/036756, WO 03/43127, WO 2004/036785, WO 2004/088790, WO 2004/102739 and WO 2005/048401. However, these do not scale up straightforwardly to two dimensions: for a two dimensional array of antenna elements arranged in rows and columns, using one of these prior art solutions per row or column permits scanning of all rows or columns in one dimension, but not scanning in another (orthogonal) dimension. The issue of scanning phased arrays is discussed in a standard work in the art of antennas, “Antenna Engineering Handbook”, Ed. Richard C. Johnson, McGraw Hill, 3rd Edition, 1993, ISBN 0-07-032381-X: see page 20-52 in particular.